1. Field of the Invention
The present invention relates to a photosensitive member for electrophotography and a fabrication process thereof and, more particularly, to a photosensitive member for electrophotography that can obtain high-quality images free of image faintness and image smearing with no heating means for photosensitive member, that has high durability enough to maintain the characteristics, that exhibits less variation in potential characteristics, that can obtain high-quality images on a stable basis, that experiences no ghost phenomenon, and to a fabrication process thereof for achieving the performance thereof with good repeatability.
2. Related Background Art
Proposed for a light receiving layer used in the photosensitive member for electrophotography are a variety of materials including selenium, cadmium sulfide, zinc oxide, phthalocyanine, amorphous silicon (hereinafter referred to as a-Si), and so on. Among others, non-monocrystalline deposited films containing silicon atoms as a main ingredient typified by a-Si, for example, amorphous deposited films of a-Si or the like compensated by hydrogen and/or halogen (for example, fluorine, chlorine, or the like), are proposed as high-performance, high-durability, and nonpolluting photosensitive members, some of which are used practically. U.S. Pat. No. 4,265,991 discloses the technology of the electrophotographic, photosensitive member the photoconductive layer of which is formed mainly of a-Si.
The a-Si photosensitive members have high surface hardness, demonstrate high sensitivity also to long-wavelength light of semiconductor lasers (770 nm to 800 nm) and the like, and exhibit little deterioration after repetitive use, and they are widely used, particularly, as photosensitive members for electrophotography in high-speed copiers, LBPs (laser beam printers), and the like.
As methods for forming such deposited films there are many conventional methods known, including the sputtering process, the method for decomposing source gas by heat (the thermal CVD process), the method for decomposing the source gas by light (the photo-CVD process), the method for decomposing the source gas by plasma (the plasma enhanced CVD process), and so on.
Among them, the plasma CVD process, which is a method for decomposing the source gas by glow discharge or the like induced by direct current, high-frequency (RF or VHF) wave, or microwave and for forming a deposited film of thin film on a substrate of glass, quartz, heat-resistive, synthetic resin film, stainless steel, aluminum, or the like, is now under very quick development into practical use as a method for forming the a-Si deposited films for electrophotography. A variety of proposals were also made on apparatus for the process.
Further, demands are becoming stronger for improvements in quality of film and in throughput and many ideas are under research.
Especially, the plasma process using high-frequency power is used because of its various advantages including high stability of discharge, applicability to formation of insulating materials such as oxide film, nitride film, and so on.
A process recently reported is the plasma enhanced CVD process with high-frequency power supply of 50 MHz or higher by use of a parallel plate plasma enhanced CVD system (Plasma Chemistry and Plasma Processing, Vol. 7, No. 3 (1987), pp. 267-273), which shows a possibility that the rate of deposition can be increased without degrading the performance of a deposited film by using the frequency of discharge higher than 13.56 MHz conventionally used and which is drawing attention. There are also reports of increasing the frequency of discharge in sputtering etc. and wide research is under way in recent years.
In applications of the a-Si photosensitive members fabricated by these methods to the electrophotographic apparatus, charging and decharging (charge-eliminating) means of photosensitive members used in the most cases are corona chargers (Corotron, Scorotron) composed of main components of a wire electrode (a metal wire such as a tungsten wire with gold plating in 50 to 100 .mu.m.phi.) and a shield plate. Specifically, charging or decharging (elimination of charge) of the surface of the photosensitive member is effected by applying a high voltage (approximately 4 to 8 kV) to the wire electrode of the corona charger and causing corona electric current generated with the application of the voltage to act on the surface of the photosensitive member. The corona chargers are excellent in uniform charging and decharging.
However, ozone (O.sub.3) is generated with corona discharge to oxidize nitrogen in the air to generate nitrogen oxides (NO.sub.x) etc. In addition, the nitrogen oxides etc. thus generated react with water in the air to generate nitric acid etc. Then the products by the corona discharge, including nitrogen oxides, nitric acid, etc., adhere to and are deposited on the photosensitive member and peripheral devices, thus polluting surfaces thereof.
Since the corona discharge products are highly hygroscopic, the surface of photosensitive member adsorbing them decreases its resistance because of absorption of moisture by the corona discharge products thus deposited, so as to substantially degrade the charge holding ability totally or partly, which is a cause to induce defects of image as called image faintness and image smearing (a pattern of electrostatic latent image is destroyed or is not formed because of leakage of charge on the surface of photosensitive member in directions along the surface).
The corona discharge products, depositing on the internal surface of shield plate in the corona charger, are volatilized and liberated not only during operation of electrophotographic apparatus, but also during rest of apparatus, for example, during the night and then adhere to the surface of photosensitive member in the region corresponding to the discharge opening of the charger. Such corona discharge products further absorb moisture to decrease the resistance of the surface of photosensitive member. Therefore, with the first copy or several copies outputted initially at restart of apparatus after rest of apparatus, the image faintness and the image smearing tend to take place in the aforementioned region corresponding to the charge opening in the rest of apparatus. This phenomenon sometimes appears prominent, especially, when the corona charger is an AC corona charger.
Particularly, when the photosensitive member is an a-Si photosensitive member, the problems of image faintness and image smearing due to the above corona discharge products become more serious. Specifically, since the a-Si photosensitive member has lower efficiency of charging and decharging than the other photosensitive members (it requires more electric current of corona charging for obtaining a predetermined charging or decharging potential), the charging current amount is increased greatly by applying a higher voltage to the charger than in the case of the other photosensitive members, in the charging and decharging processes by corona discharge to the a-Si photosensitive member.
Since an amount of ozone generated is in proportional relation with the current amount of corona charging, in the case of the arrangement wherein the photosensitive member is the a-Si photosensitive member and wherein it is charged or decharged by corona charging, the amount of ozone generated particularly becomes large, which could make the problems of image faintness and image smearing due to generation of the aforementioned corona discharge products particularly serious in some cases. In addition, in the case of the a-Si photosensitive member, the surface hardness thereof is extremely higher than those of the other photosensitive members, and the extremely high surface hardness inversely acts to keep the corona discharge products on the surface of photosensitive member, remaining indefinitely.
The following two methods are considered to be methods for preventing the phenomena of image faintness and image smearing.
(1) The first method is a method for decreasing relative humidity by heating the surface of the photosensitive member (up to 30 to 50.degree. C.) by a heater for heating the photosensitive member built in the photosensitive member or by hot air supplied from a hot wind supplying device to the photosensitive member. This method is a process for preventing the substantial decrease in resistance of the surface of photosensitive member by volatilizing the corona discharge products and water adhering to the surface of photosensitive member and this method is in practical use.
(2) The second method is a method for increasing the water repellency of the surface so as to make the corona discharge products hard to adhere to the surface from the beginning, thereby preventing the image smearing. For example, the bulletin of Japanese Patent Application Laid-Open No. 61-289354 discloses an a-C surface layer a surface of which is plasma-processed with a gas containing fluorine. Further, the bulletin of Japanese Patent Application Laid-Open No. 64-84257 discloses an electrophotographic, photosensitive member wherein the surface of a surface layer comprised of amorphous carbon containing fluorine has unevenness 0.1 to 0.5 .mu.m deep and 0.1 to 1 .mu.m wide, and a fabrication process thereof. It is described that this method can further increase water repellency by increasing the surface area.
While the problem of image smearing was solved by the heating device of a drum in the above-stated method of (1), new problems described below arose with recent growing demands for copiers. Namely, circumstances have been changing so that use of the heating means by drum heater should be preferably avoided from the viewpoints of energy saving and ecology.
Another problem arises when the a-Si drum assuring high quality of image is mounted in a full-color copier. The problem is to increase a possibility of fusing and sticking low-melting-point toner of color toner or the like onto the surface of drum. Still another problem is that image densities change partly depending upon cycles of rotation of a rotary, cylindrical developer-carrying member.
The reason is that during rest of the apparatus the heat of the photosensitive member expands the rotary, cylindrical developer-carrying member to decrease the distance to the portion of photosensitive member opposed thereto, whereby the developer becomes easier to transfer than in the normal condition. These problems raised demands for a photosensitive member free from occurrence of image faintness and image smearing without heating.
On the other hand, as to the technology for increasing water repellency as disclosed in the aforementioned bulletins in the above-stated method of (2), the bulletins describe the increase in water repellency by exposure to ozone, but they describe nothing about whether durability tests were conducted or not by carrying out a large number of copying operations in actual.
Then the inventors conducted follow-up tests according to the method disclosed in the bulletin of Japanese Patent Application Laid-Open No. 61-289354 to carry out durability tests of performing continuous copying operations without use of heating means. The durability tests verified that durability was insufficient, though the image smearing characteristics at an initial stage were improved in fact. It was found that a property of being capable of maintaining necessary and sufficient water repellency even after sheet-pass durability tests of many sheets was more preferable in consideration of the high durability as one of merits of the a-Si photosensitive member.
The inventors also conducted follow-up tests of the method disclosed in the bulletin of Japanese Patent Application Laid-Open No. 64-84257. In this case, it was confirmed that increasing the surface area by providing the surface with unevenness was effective in increasing water repellency, but it was also found that the effect of water repellency decreased with progress of sheet-pass durability test. A reason of this is possibly that amorphous carbon containing fluorine is soft and thus is shaved gradually by slip abrasion with members around the photosensitive member, paper, or the like.
Under these circumstances, the desire has been growing for a photosensitive member that has a surface layer with high water repellency so as not to cause the image faintness or the image smearing without use of heating means and that exhibits no degradation of the high water repellency even after copying operations of many sheets for a long period.
With recent increase in the requirements for copy images, in addition to the problem of image smearing, there is earnest desire for the technology for supplying high-quality images on a stable basis. Since uses of copier have been changing from copy originals mainly containing letters to images of photographs or the like to increase copy originals frequently using halftones, a severer standard than before is required for stability of density. When potentials of charging vary with time against the desire to keep charging equal as described above, the change of potential is called a potential shift, and the smaller the potential shift, the higher the stability.
With photographic originals, the requirement becomes severer as to the ghost phenomenon as well as the stability of density. This is because halftone portions are likely to be affected especially by ghost. The "ghost" herein is a phenomenon that an image in previous copying operation is visualized as a light image. A difference between a potential in a portion where the ghost appears upon application of a halftone potential and the halftone potential is called as a ghost potential, and the smaller the ghost potential, the less the ghost appears.
A reason why these occur is possibly that the charge is trapped in the film for some reason in the process of carrying out charging or decharging. Attempts have been made to decrease the potential shift and ghost potential by various methods, but the technology capable of achieving a further improvement is desired earnestly.